The IMS is an IP-based network architecture proposed by the 3rd Generation Partnership Project (3GPP), which constructs an open and flexible service environment, supports multimedia applications, and can provide rich multimedia services for a user.
In an IMS service hierarchy, a control layer is separate from a service layer, and the control layer does not provide any specific service, but only provides the service layer with necessary functions such as triggering, routing, charging, and the like.
The service triggering and controlling functions in the control layer are performed by a Call Session Control Function (CSCF). There are mainly three types of CSCF, namely, Proxy-CSCF (P-CSCF), Interrogating-CSCF (I-CSCF), and Serving-CSCF (S-CSCF), wherein the S-CSCF is required, while the I-CSCF is optional.
The service layer consists of a series of Application Servers (AS) and can provide a specific service, wherein the ASs can be independent entities or exist in the S-CSCF.
The control layer (S-CSCF) controls service triggering according to subscription information of a user to invoke a service on an AS to implement a service function.
The end-to-end equipment in a session is called a UE, and is responsible for interaction with the user of the UE. The UE has multiple ways to access a network, including: accessing via a 3GPP PS domain, or accessing via another non-3GPP PS domain, or even accessing via a CS domain, etc. When an IMS user connects from a CS domain and initiates a call, the call signaling thereof needs to be routed to an IMS domain home to the user, so as to provide a service to the user.
If at a certain moment, the UE can only use either a CS domain or a PS domain, instead of simultaneously using both, for providing a voice service, a handover to the PS domain occurs when the UE uses the CS domain for providing the voice service, and when it is required to provide the voice service using the PS domain, it is called a Reverse Single Radio Voice Call Continuity (RSRVCC).
As a UE may still use a PS domain for providing another non-voice service when using the CS domain for providing the voice service, a PS domain non-voice service handover may still occur while the RSRVCC occurs. As services are provided to the UE at a source network respectively by a CS domain and a PS domain, and after handover, services are provided to the UE only by a PS domain in a target network, it certainly is necessary to synchronize CS handover and PS handover at the time of handover, namely, to synchronize handover from a CS domain to a PS domain, and that from a PS domain to a PS domain.